Incidents have been reported concerning the loss of residual heat removal system (RHRS) operation due to human errors, equipment malfunctions and loss of cooling fluid inventory in the reactor coolant system. Among the incidents reported, some have occurred because the cooling fluid level in the reactor coolant system was drained too low. When the reactor coolant system (RCS) water level is drained below a certain level, vortex begins to form and the residual heat removal (RHR) pumps are cavitated or become airbound. The RHR pumps must be manually stopped to prevent damage. In current PWR plants these pumps are safety injection pumps. Consequently, decay heat removal and low head safety injection functions are lost, resulting in RCS heatup and potential core uncovery. An extended period of core uncovery would cause fuel damage.
The time margin available for restoring the RHRS, or establishing alternate methods of heat removal (prior to bulk boiling, core uncovery, fuel damage, etc.) depends on the RCS temperature, the decay heat rate (which is dependent on time interval elapsed from reactor trip to RHRS failure and core power operation history), and the amount of RCS inventory.
One of the worst cases would be the loss of RHRS during mid-loop operation. Mid-loop operation is an operation where the RCS is partially drained to approximately the mid-level of the hot leg to perform steam generator inspection or repairs. The RHR pump operates at its designated flow rate and the RCS inventory is reduced. If the RCS inventory drops below a certain level (which may occur rapidly since there is little level margin in the RCS), vortex begins to form and the RHR pump cavitates, resulting in a loss of RHRS. The reduced RCS inventory reduces the time available to recover the RHRS prior to bulk boiling and core uncovery.
An improved such pressurized water nuclear reactor system differs from traditional PWR design in that it does not have an independent RHR system. An example of such an improved system is described in U.S. No. 4,769,209, issued Sept. 6, 1988, and assigned to the assignee of the present invention, the contents of said patent being incorporated by reference herein. The traditional decay heat removal function combines with the spent fuel cooling function to form a spent cooling system in such an improved system. One advantage of such a combination is that the improved pressurized water nuclear reactor system spent fuel cooling system can perform a spent fuel pit cooling and decay heat removal simultaneously and independently. Because the two identical pump and heat exchanger trains can be used interchangeably, a RHR capacity redundancy requirement is fulfilled. The other advantage is that the pumps are no longer safety injection pumps.
However, the loss of RHR cooling problem associated with the mid-loop operation in traditional plants still applies to this improved pressurized water reactor system. In the improved system, the steam generator inspection and repair requires that the hot leg water level be drained to a certain level. Since the decay heat removal pump takes suction from the hot leg while the mid-loop operation is in progress, too low a hot leg level can induce vortex and cavitate the pump, resulting in similar consequences as in traditional PRW plants.
It is an object of the present invention to provide a decay heat removal system, containing a pump, in a pressurized water nuclear reactor that will minimize or eliminate vortex induced cavitation or air introduction into the pump of the system.